This invention generally relates to chemical mechanical polishing (CMP) and more particularly to a method for maintaining and measuring a dielectric layer thickness wafer reference standard for improved metrology calibration.
In semiconductor fabrication integrated circuits and semiconducting devices are formed by sequentially forming features in sequential layers of material in a bottom-up manufacturing method. The manufacturing process utilizes a wide variety of techniques to form the various layered features including various deposition techniques and thermal growth techniques. For example, in the manufacture of a metal oxide semiconductor (MOS) device, a thin dielectric layer, is formed on a surface of the semiconductor substrate. The dielectric layer is for example, silicon dioxide or silicon nitride, which is typically used as a gate dielectric for the MOS device. To assure proper functioning of the device, quality control measures are carried out to insure the thickness of the dielectric layer is within a specified range.
One problem associated with thin dielectric layers is that upon exposure of the dielectric layer, to an ambient environment, a contamination layer including for example, contamination and/or an oxide layer, is formed and grows over time on the dielectric layer at a given rate depending on, for example, the humidity level and contamination level in the ambient environment. Various approaches have been proposed to control the growth of the dielectric layer or to accurately predict its growth rate. A recurring problem in all these approaches however is the need for precision and accuracy in the dielectric layer thickness metrology tool. For example, if the accuracy of the metrology tool cannot be relied upon to a high degree, the occurrence of layer growth cannot be reliably determined.
For example, optical metrology tools are frequently used for determining dielectric layer thickness where various components of reflected light are analyzed to determine a dielectric layer thickness. For example, optical measurement devices, including a Beam Profile Reflectometer (BPR), a Beam Profile Ellipsometer (BPE), and a Broadband Reflective Spectrometer (BRS) are examples of optical metrology tools. Each of these devices measures parameters of optical beams reflected by, or transmitted through, the target sample.
The accuracy of dielectric layer thickness measurements depends on calibration of the metrology tool by providing a reference sample having a known substrate including a thin dielectric layer of known composition and thickness. For example, the metrology tool is periodically used to measure the reference sample and appropriately calibrated based on the known thickness of the reference sample. Typically a reference sample includes a xe2x80x9cnative oxidexe2x80x9d reference sample, which is a silicon substrate with a silicon dioxide layer formed thereon having a thickness of about 20 Angstroms or less. The reference sample is stored in a controlled ambient environment to minimize oxide layer growth and contamination. In many cases, for example, with an Ellipsometer, industry measurement standards require several calibration measurements a day to account for oxide growth or contamination on the reference sample. The instability of the reference sample results in the frequent change of reference samples requiring more time consuming recalibration and presents serious quality control problems in device performance.
In the prior art, where dielectric layers included dimensions of, for example, greater than 50 Angstroms, a moderate amount of oxide growth and contamination was tolerable since the accuracy of the calibration was still within acceptable limits. However, the requirement for accurate measurements of dielectric layers less than about 20 Angstroms has required a higher degree of dielectric thickness measurement accuracy and consequently more exacting calibration requirements. For example, newer optical metrology tools can measure dielectric layers on the order of 10 Angstroms, requiring a high degree of stability in the reference sample for acceptable calibration.
Another problem with prior art methods using reference samples for calibration is that thickness measurements vary outside the limits of acceptable accuracy between different thickness metrology tools. For example, one metrology tool used for one manufacturing process using a particular reference sample frequently gives different results on another metrology tool making it difficult to determine which metrology tool is off calibration and whether the discrepancy is due to the reference sample or other factors attributable to the metrology tools.
For example, referring to FIG. 1 is shown a graph 10 of an exemplary series of daily measurements over time of a wafer calibration reference standard with a starting silicon dioxide layer thickness of about 20 Angstroms. The vertical axis is the silicon dioxide layer thickness in Angstroms, while the horizontal axis is time in days. Line 12A represents oxide layer thickness spot measurements taken over time at the same central portion of the reference wafer (e.g., position 0, 0) together with regression line fit 12B while line 14A represents oxide layer thickness spot measurements taken over time at the same off-center (displaced from center) portion of the reference wafer (e.g., position 0, 47) together with regression line fit 14B. The measurements were made with a Rudolph SL-200 ellipsometer with a spot size of about 7xc3x9712 microns. No pre-measurement cleaning of the calibration reference standard was performed consistent with prior art calibration methods. The results are exemplary measurements demonstrating that the entire surface of the calibration reference standard including the silicon dioxide layer has a similar oxide and/or contamination growth rate.
The trend in dielectric layer (e.g., silicon dioxide) thickness growth of the calibration reference standard presents a serious problem in quality control monitoring for semiconductor processing technologies including for example, 0.13 micron features and below, and gate oxide thicknesses on the order of 15 to 20 Angstroms. Inaccurate dielectric layer thickness measurements can result in unnecessary changes in production line processes and unnecessary scraping of production batches.
Therefore, there is a need in the semiconductor art to develop a method for achieving reliable metrology tool calibration for measuring dielectric layer thicknesses including calibration reference standards in order to accurately measure thin dielectric layer thicknesses and achieve calibration consistency between different metrology tools.
It is therefore an object of the invention to provide a method for achieving reliable metrology tool calibration for measuring dielectric layer thicknesses including calibration reference standards in order to accurately measure thin dielectric layer thicknesses and achieve calibration consistency between different metrology tools while overcoming other shortcomings and deficiencies in the prior art.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides a method for measuring a dielectric layer thickness calibration reference standard.
In a first embodiment, the method includes providing a substrate having a dielectric layer for calibrating a dielectric layer thickness measuring tool; cleaning the dielectric layer according to a cleaning process including at least one of spraying and scrubbing; and, measuring the thickness of the dielectric layer with the dielectric layer thickness measuring tool including at least one portion of the dielectric layer displaced from the substrate center.